1. Field of the Invention
The present invention relates to a one-liquid type epoxy resin composition which can be stored for extended periods of time at ordinary temperature and cures quickly upon heating.
2. Prior Art
Epoxy resins of the two-liquid type have heretofore been used being comprised of an epoxy resin and a curing agent. The epoxy resin composition of the two-liquid type, however, require such operations as weighing, stirring and mixing at a time when they are used on the site. These operations involve such problems as a decreased operation efficiency, erroneous weighing, or disposal of excess amounts of the epoxy resin composition. At present, therefore, study has been extensively conducted in an effort to developing one-liquid type epoxy resin compositions that permit highly efficient workability on the site and can be stored for extended periods of time to substitute for the two-liquid type ones.
Development of a latent epoxy curing agent is a prerequisite for preparing the one-liquid type epoxy resin compositions. Known examples of the latent epoxy curing agent include dicyandiamide, carboxylic acid hydrazide, guanamine compounds, imidazole compounds. However, these latent curing agents cure the composition by heating at relatively high temperatures for extended periods of time. It has therefore been urged to provide a curing agent that activates at a low temperature and exhibits an excellent latent property.
As latent curing agents for curing epoxy resins at relatively low temperatures, there have been known an adduct of an alkylamine-and an epoxy resin, and an adduct of an imidazole compound and an epoxy resin (U.S. Pat. No. 4,066,625). These adducts only, however, are not enough for obtaining latency to a satisfactory degree. Therefore, additional modifications have been reported in which the surfaces thereof are coated with various compounds. For instance, there have been reported a method of encapsulation with an isocyanate compound (Japanese Laid-Open Patent Publication No. 268723/1986) and a method of surface treatment using an acidic substance (Japanese Patent Publication No. 18826/1990, Japanese Laid-Open Patent Publication No. 223027/1988).
However, the object has not yet been achieved to a sufficient degree by the above-mentioned attempts of reliably obtaining the latency by the surface treatment. For instance, the capsule film of the isocyanate compound is easily destroyed by the mechanical impact but is not sufficiently regenerated. Besides, the treatment with an acidic compound such as an organic acid, an inorganic acid, a resin acid or an aluminum compound does not help obtain stabilized latency.